1. Field of the Invention
The present invention relates to a magnetic memory cell in which information writing and information reading are performed by the use of a magnetoresistance effect and also relates to a device using the same.
2. Description of the Related Art
The magnetoresistance effect is a phenomenon that an electric resistance is changed when a magnetic field is applied to a magnetic material or when a magnetization state of a magnetic material is changed. A magnetic head and a magnetic sensor have been known as a magnetoresistance effect device utilizing the magnetoresistance effect. Moreover, recently, a nonvolatile solid-state magnetic memory device (MRAM) and the like are being experimentally manufactured.
A mainstream configuration of an MRAM which is now experimentally manufactured has a matrix structure in which tunnel magnetoresistance (TMR) elements are arranged at intersection points between bit lines and word lines. A magnetization direction of each of the TMR elements is reversed by a synthetic magnetic field formed by a current flowing through each line. Thus, the writing of information is performed. In this case, the TMR element serves as a memory cell. In order to read information from the memory cell, cell selection by a MOS transistor is required due to the presence of a leak current from the memory cell. As to the MRAM having the above structure, there have been pointed out, due to the structure, a drawback such as a complex process technology as well as the following three drawbacks that make the MRAM unfit for having higher density and larger capacity. The first drawback is that effective reversal magnetic field conditions are narrowed by reduction in size of the memory cell. The second is that a reversal magnetic field is increased by reduction in thickness of the magnetic material, and therefore a line current and power consumption are increased. The third is that, having a MOS transistor, the MRAM has a possible integration level only as high as a DRAM.
In the meantime, there has recently been proposed a spin torque, which is a magnetization reversal process using no reversal magnetic field, and the occurrence of magnetization reversal by the spin torque has actually been confirmed. It has also been proposed that the spin torque be used for writing into a memory cell. However, the magnetization reversal by use of the spin torque currently has the following technical problem. The problem is specifically that a critical current density as high as approximately 108 A/cm2 is required to generate the spin torque that causes the magnetization reversal.
Meanwhile, spin torque magnetization reversal caused by a spin flow alone has been examined by use of a device structure called a non-local spin valve. However, it has only been confirmed that magnetization of a magnetization free layer is rotated to one direction from an anti-parallel state to a parallel state with respect to a magnetization pinned layer. Thus, writing into a memory cell with perfect control has not yet been realized; see Phys. Rev. Lett. 96, 037201-1-037201-4 (2006) (Non-patent Document 1). When only one direction of rotation was confirmed, the value of a critical current density in a non-magnetic portion that carries the spin flow became as high as approximately 2×108 A/cm2.
Since the TMR element utilizes a high resistance of a tunnel barrier, such a high critical current density required for magnetization reversal leads to a concern over electrostatic breakdown of the tunnel barrier. In order to avoid the electrostatic breakdown, it has been proposed to separate a write current path from a read current path in a memory cell by making an element including three terminals in Japanese Patent Application Laid-Open Publication No. 2006-156477. However, since this three-terminal device structure includes three magnetic layers on a read current path, there arises a concern over a magnetic noise effect as the device is miniaturized.